The present invention relates to a high carbon steel sheet having chemical composition specified by JIS G 4051 (Carbon steels for machine structural use), JIS G 4401 (Carbon tool steels) or JIS G 4802 (Cold-rolled steel strips for springs), and in particular to a high carbon steel sheet having excellent hardenability and toughness, and workability with a high dimensional precision, and a method of producing the same.
High carbon steel sheets having chemical compositions specified by JIS G 4051, JIS G 4401 or JIS G 4802 have conventionally much often been applied to parts for machine structural use such as washers, chains or the like. Such high carbon steel sheets have accordingly been demanded to have good hardenability, and recently not only the good hardenability after quenching treatment but also low temperaturexe2x80x94short time of quenching treatment for cost down and high toughness after quenching treatment for safety during services. In addition, since the high carbon steel sheets have large planar anisotropy of mechanical properties caused by production process such as hot rolling, annealing and cold rolling, it has been difficult to apply the high carbon steel sheets to parts as gears which are conventionally produced by casting or forging, and demanded to have workability with a high dimensional precision.
Therefore, for improving the hardenability and the toughness of the high carbon steel sheets, and reducing their planar anisotropy of mechanical properties, the following methods have been proposed.
(1) JP-A-5-9588, (the term xe2x80x9cJP-Axe2x80x9d referred to herein signifies xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d) (Prior Art 1): hot rolling, cooling down to 20 to 500xc2x0 C. at a rate of 10xc2x0 C./sec or higher, reheating for a short time, and coiling so as to accelerate spheroidization of carbides for improving the hardenability.
(2) JP-A-5-98388 (Prior Art 2): adding Nb and Ti to high carbon steels containing 0.30 to 0.70% of C so as to form carbonitrides for restraining austenite grain growth and improving the toughness.
(3) xe2x80x9cMaterial and Processxe2x80x9d, vol.1 (1988), p.1729 (Prior Art 3): hot rolling a high carbon steel containing 0.65% of C, cold rolling at a reduction rate of 50%, batch annealing at 650xc2x0 C. for 24 hr, subjecting to secondary cold rolling at a reduction rate of 65%, and secondary batch annealing at 680xc2x0 C. for 24 hr for improving the workability; otherwise adjusting the chemical composition of a high carbon steel containing 0.65% of C, repeating the rolling and the annealing as above mentioned so as to graphitize cementites for improving the workability and reducing the planar anisotropy of r-value.
(4) JP-A-10-152757 (Prior Art 4): adjusting contents of C, Si, Mn, P, Cr, Ni, Mo, V, Ti and Al, decreasing S content below 0.002 wt %, so that 6 xcexcm or less is the average length of sulfide based non metallic inclusions narrowly elongated in the rolling direction, and 80% or more of all the inclusions are the inclusions whose length in the rolling direction is 4 xcexcm or less, whereby the planar anisotropy of toughness and ductility is made small.
(5) JP-A-6-271935 (Prior Art 5): hot rolling, at Ar3 transformation point or higher, a steel whose contents of C, Si, Mn, Cr, Mo, Ni, B and Al were adjusted, cooling at a rate of 30xc2x0 C./sec or higher, coiling at 550 to 700xc2x0 C., descaling, primarily annealing at 600 to 680xc2x0 C., cold rolling at a reduction rate of 40% or more, secondarily annealing at 600 to 680xc2x0 C., and temper rolling so as to reduce the planar shape anisotropy caused by quenching treatment.
However, there are following problems in the above mentioned prior arts.
Prior Art 1: Although reheating for a short time, followed by coiling, a treating time for spheroidizing carbides is very short, and the spheroidization of carbides is insufficient so that the good hardenability might not be probably sometimes provided. Further, for reheating for a short time until coiling after cooling, a rapidly heating apparatus such as an electrically conductive heater is needed, resulting in an increase of production cost.
Prior Art 2: Because of adding expensive Nb and Ti, the production cost is increased.
Prior Art 3: xcex94r=(r0+r90xe2x88x922xc3x97r45)/4 is xe2x88x920.47, which is a parameter of planar anisotropy of r-value (r0, r45, and r90 shows a r-value of the directions of 0xc2x0 (L), 45xc2x0 (S) and 90xc2x0 (C) with respect to the rolling direction respectively). xcex94max of r-value being a difference between the maximum value and the minimum value among r0, r45, and r90 is 1.17. Since the xcex94r and the xcex94max of r-value are high, it is difficult to carry out a forming with a high dimensional precision.
Besides, by graphitizing the cementites, the xcex94r decreases to 0.34 and the xcex94max of r-value decreases to 0.85, but the forming could not be carried out with a high dimensional precision. In case graphitizing, since a dissolving speed of graphites into austenite phase is slow, the hardenability is remarkably degraded.
Prior Art 4: The planar anisotropy caused by inclusions is decreased, but the forming could not be always carried out with a high dimensional precision.
Prior Art 5: Poor shaping caused by quenching treatment could be improved, but the forming could not be always carried out with a high dimensional precision.
The present invention has been realized to solve above these problems, and it is an object of the invention to provide a high carbon steel sheet having excellent hardenability and toughness, and workability with a high dimensional precision, and a method of producing the same.
The present object could be accomplished by a high carbon steel sheet having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, in which the ratio of number of carbides having a diameter of 0.6 xcexcm or less with respect to all the carbides is 80% or more, more than 50 carbides having a diameter of 1.5 xcexcm or larger exist in 2500 xcexcm2 of observation field area of electron microscope, and the xcex94r being a parameter of planar anisotropy of r-value is more than xe2x88x920.15 to less than 0.15.
The above mentioned high carbon steel sheet can be produced by a method comprising the steps of: hot rolling a steel having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, coiling the hot rolled steel sheet at 520 to 600xc2x0 C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at 640 to 690xc2x0 C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or more, and secondarily annealing the cold rolled steel sheet at 620 to 680xc2x0 C.
The JIS G standards JIS G 4051 (1979), JIS G 4401:2000 and JIS G 4802:1999 and particularly the section of each disclosing the chemical composition, are hereby incorporated by reference.